This invention concerns thermoforming in which plastic sheet material is preheated and then advanced over a mold cavity plate, and then drawn into the mold cavities by the application of differential fluid pressure to the sheet material, as by development of a vacuum within the cavities and/or a positive pressure on the sheet material on the opposite side from the cavities. Proper stretching of the sheet material is critical to producing a properly formed part, as too much stretching in localized areas can reduce the wall thickness to an unacceptable degree. An unsightly appearance of the finished product can result from nonuniform stretching of the sheet material. Sophisticated tooling has been developed to insure proper stretching of the sheet material including clamping rings tightly engaging the sheet material surrounding each mold cavity, which are forced against the sheet material prior to thermoforming the parts. This limits the stretching of sheet material to the section secured within the clamping ring.
Another widely practiced thermoforming technique, particularly when making deep drawn parts such as disposable cups, involves the used of “plug assists”, which are plug shaped members advanced into the sheet material to force it into the mold cavity prestretch the material by movement into the mold cavity prior to forming the part so as to prevent localized over stretching.
Another critical aspect of thermoforming is preventing localized cooling of the material as by contact with meal surfaces which could chill the sheet material at the point of contact, preventing proper stretching of the material around the point of contact.
The plug assists are typically constructed of syntactic foam to prevent chilling by contact of the sheet material therewith.
When thermoforming disposable drink cups, a lip is formed on the upper edge to produce a finished look. The lip is partially formed during molding by a projecting ring on the mold cavity with the sheet wrapped around the rim during thermoforming. The lip is rolled under in a subsequent lip rolling operation. Early contact with the lip ring is apt to occur during plunging of the plug assists, and this contact can cause a slight chilling of the sheet material at the point of contact, forming an irregular ridge in the sidewall as the chilled portion is drawn down into the cavity during the plug assist plunge. This is particularly apt to occur when thermoforming from thin gauge plastic fed off a roll at high speed.
Such a ridge is unsightly, particularly in clear plastic cups, and can make the release of the cups from a stack more difficult. The ridge also can interfere with printing on the cup side wall.
In an effort to avoid this problem, the projecting edge has been recessed into a “moat” feature, but this has led to difficulties in trimming, since the trim tooling must reach into the moat. Furthermore, additional sheet material is used up in the molding operation since a larger clamped circle is necessitated.
It is an object of the present invention to provide a thermoforming method and apparatus which avoids contact of the heated sheet material with lip ring edge of the mold cavity during a plug assist plunge but does not involve the use of “moats” to recess the lip ring edge.